Organochlorines such as DDT, its metabolite DDE, and polychlorinated biphenyls (PCBs) are persistent organic pollutants that may be toxic at levels encountered by the general population, through exposure in the food chain. Each organochlorine has one or more types of biologic activity, and corresponding potential mechanisms of action. Last year's progress I collaborated with investigators who had studied pregnancies among a population with relatively high exposure to DDT in China. We published a study on the relation of DDT and its metabolite DDE to reproductive outcomes. Our results provide suggestive evidence that B12 and folate sufficiency may help protect against adverse reproductive effects of DDT exposure.